The proposed research involves obtaining His bundle electrograms for patients with bundle branch block and following these patients in order to determine the incidence of subsequent progression to 2 degrees or 3 degrees AV block. Our primary objective is to correlate measurement of infranodal conduction time with these target events. In addition, we plan to follow those patients with bundle branch block who have undergone prophylactic permanent pacemaker insertion in order to determine whether cardiac pacing affects the incidence of sudden cardiac death in this patient cohort. We are also obtaining repeat His bundle recordings in order to assess the progression rate of infranodal conduction disturbances in patients with bundle branch block. Finally, we propose to assess the effects of commonly used antiarrhythmic agents in patients with both chronic bundle branch block as well as those with bundle branch block due to acute myocardial infarction.